Industrial wastewater from plants is subjected to purifying treatments, such as elimination of heavy metal components, suspended particles, and the like and decomposition and elimination of organic matter using microorganisms. Although the treated water subjected to the purifying treatment is generally discharged into environment, the treated water obtained by subjecting the industrial wastewater to the purifying treatment is recycled for industrial water purpose in places where securing industrial water is difficult.
As methods of decomposing and eliminating organic matter by microorganisms, a membrane-separated activated sludge method (membrane bioreactor (MBR)) and a biofilm method (biofilm reactor (BFR)) are used.
The membrane-separated activated sludge method is a method of decomposing organic matter in wastewater using activated sludge within a tank, and separating treated water and the activated sludge using a filtration membrane. Since the membrane-separated activated sludge method can make the concentration of the activated sludge high and make a settling tank separating the treated water and the activated sludge unnecessary, there is an advantage of volumetric efficiency being high. Meanwhile, if water quality, that is, organic matter concentration in the wastewater fluctuates largely in a short period of time, treatment capacity cannot follow fluctuations of the organic matter concentration, and the organic matter concentration in the treated water may exceed a control value.
The biofilm method is a method of making the activated sludge adhere to a carrier surface in the shape of a film, and decomposing the organic matter in the wastewater. Although the same treatment as the usual activated sludge is performed on the film (biofilm) surface, since a source of nutrition is insufficient in the inner surface of the film, the organic matter is brought into a dormant state. When the organic matter concentration fluctuates, the sludge inside the biofilm is activated. That is, the thickness of active sludge fluctuates according to fluctuations of the organic matter concentration. Since the amount of the activated sludge according to the organic matter concentration is maintained, it is possible to continue the treatment corresponding to the fluctuations of the organic matter concentration.
In wastewater treatment devices, generally, any one of the membrane-separated activated sludge method and the biofilm method is applied. PTL 1 discloses a water treatment device that includes a biological treatment tank and that can cope with a case where the time-series fluctuations of the flow rate of the water to be treated are large. In the water treatment device of PTL 1, the water before treatment is taken into a flow rate regulating tank, and the flow rate of the water sent to biological treatment means and the water sent to ozonizing means is controlled according to the pollution loading amount of the water in the flow rate regulating tank.